By employing the second table (by the method of interpolation) the specific gravity, at a given temperature (from 0° to 30°) can be found for any percentage amount of H2SO4, and therefore conversely the percentage of H2SO4 can be found from the specific gravity.
[55 bis] Whether similar (even small) breaks in the continuity of the factor dS/dp exist or not, for other hydrates (for instance, for H2SO4H2O and H2SO44H2O) cannot as yet be affirmed owing to the want of accurate data (Note [53]). In my investigation of this subject (1887) I admit their possibility, but only conditionally; and now, without insisting upon a similar opinion, I only hold to the existence of a distinct break in the factor at H2SO4, being guided by C. Winkler's observations ond the specific gravities of fuming sulphuric acid.
[56] In 1887, on considering all the existent observations for a temperature 0°, I gave the accompanying scheme (p. [243]) of the variation of the factor ds/dp at 0°.
I did not then (1887) give this scheme an absolute value, and now after the appearance of two series of new determinations (Lunge and Pickering in 1890), which disagree in many points, I think it well to state quite clearly: (1) that Lunge's and Pickering's new determinations have not added to the accuracy of our data respecting the variation of the specific gravity of solutions of sulphuric acid; (2) that the sum total of existing data does not negative (within the limit of experimental accuracy) the possibility of a rectilinear and broken form for the factors ds/dp; (3) that the supposition of ‘special points’ in ds/dp, indicating definite hydrates, finds confirmation in all the latest determinations; (4) that the supposition respecting the existence of hydrates determining a break of the factor ds/dp is in in way altered if, instead of a series of broken straight lines, there be a continuous series of curves, nearly approaching straight lines; and (5) that this subject deserves (as I mentioned in 1887) new and careful elaboration, because it concerns that foremost problem in our science—solutions—and introduces a special method into it—that is, the study of differential variations in a property which is so easily observed as the specific gravity of a liquid.
[56 bis] These hydrates are: (a) H2SO4 = SO3H2O (melts at + 10°·4); (b) H2SO4H2O = SO32H2O (crystallo-hydrate, melts at +8°·5); (c) H2SO42H2O (is apparently not crystallisable); (d) one of the hydrates between H2SO46H2O and H2SO43H2O, most probably H2SO44H2O = SO35H2O, for it crystallises at -24°·5 (Note [50 bis]); and (e) a certain hydrate with a large proportion of water, about H2SO4150H2O. The existence of the last is inferred from the fact that the factor ds/dp first falls, starting from water, and then rises, and this change takes place when p is less than 5 p.c. Certainly a change in the variation of ds/dp or ds/dt does take place in the neighbourhood of these five hydrates (Pickering, 1890, recognised a far greater number of hydrates). I think it well to add that if the composition of the solutions be expressed by the percentage amount of molecules—r1SO3 + (100 - r1)H2O we find that for H2SO4, r1 = 50, for H2SO42H2O r1 = 25 = 50/2, for H2SO4H2O, r1 = 33·333 = 50·⅔, while for H2SO44H2O, r1 = 16·666 = 50·⅓—i.e. that the chief hydrates are distributed symmetrically between H2O and H2SO4. Besides which I may mention that my researches (1887) upon the abrupt changes in the factor for solutions of sulphuric acid, and upon the correspondence of the breaks of ds/dp with definite hydrates, received an indirect confirmation not only in the solutions of HNO3, HCl, C2H6O, C3H8O, &c., which I investigated (in my work cited in Chapter I., Note [19]), but also in the careful observations made by Professor Cheltzoff on the solutions of FeCl3 and ZnCl2 (Chapter XVI., Note [4]) which showed the existence in these solutions of an almost similar change in ds/dp as is found in sulphuric acid. The detailed researches (1893) made by Tourbaba on the solutions of many organic substances are of a similar nature. Besides which, H. Crompton (1888), in his researches on the electrical conductivity of solutions of sulphuric acid, and Tammann, in his observations on their vapour tension, found a correlation with the hydrates indicated as above by the investigation of their specific gravities. The influence of mixtures of a definite composition upon the chemical relations of solutions is even exhibited in such a complex process as electrolysis. V. Kouriloff (1891) showed that mixtures containing about 3 p.c., 47 p.c. and 73 p.c. of sulphuric acid—i.e. whose composition approaches that of the hydrates H2SO4150H2O, H2SO46H2O and H2SO42H2O—exhibit certain peculiarities in respect to the amount of peroxide of hydrogen formed during electrolysis. Thus a 3 p.c. solution gives a maximum amount of peroxide of hydrogen at the negative pole, as compared with that given by other neighbouring concentrations. Starting from 3 p.c., the formation of peroxide of hydrogen ceases until a concentration of 47 p.c. is reached.
[57] Cellulose, for instance unsized paper or calico, is dissolved by strong sulphuric acid. Acid diluted with about half its volume of water converts it (if the action be of short duration) into vegetable parchment (Chapter I., Note [18]). The action of dilute solutions of sulphuric acid converts it into hydro-cellulose, and the fibre loses its coherent quality and becomes brittle. The prolonged action of strong sulphuric acid chars the cellulose while dilute acid converts it into glucose. If sulphuric acid be kept in an open vessel, the organic matter of the dust held in the atmosphere falls into it and blackens the acid. The same thing happens if sulphuric acid be kept in a bottle closed by a cork; the cork becomes charred, and the acid turns black. However, the chemical properties of the acid undergo only a very slight change when it turns black. Sulphuric acid which is considerably diluted with water does not produce the above effects, which clearly shows their dependence on the affinity of the sulphuric acid for water. It is evident from the preceding that strong sulphuric acid will act as a powerful poison; whilst, on the other hand, when very dilute it is employed in certain medicines and as a fertiliser for plants.
[58] Weber (1884) obtained a series of salts R2O,8SO3nH2O for K, Rb, Cs, and Tl.
[58 bis] Ditte (1890) divides all the metals into two groups with respect to sulphuric acid; the first group includes silver, mercury, copper, lead, and bismuth, which are only acted upon by hot concentrated acid. In this case sulphurous anhydride is evolved without any by-reactions. The second group contains manganese, nickel, cobalt, iron, zinc, cadmium, aluminium, tin, thallium, and the alkali metals. They react with sulphuric acid of any concentration at any temperature. At a low temperature hydrogen is disengaged, and at higher temperatures (and with very concentrated acid) hydrogen and sulphurous anhydride are simultaneously evolved.
[59] For example, the action of hot sulphuric acid on nitrogenous compounds, as applied in Kjeldahl's method for the estimation of nitrogen (Volume I. p. [249]). It is obvious that when sulphuric acid acts as an oxidising agent it forms sulphurous anhydride.
The action of sulphuric acid on the alcohols is exactly similar to its action on alkalis, because the alcohols, like alkalis, react on acids; a molecule of alcohol with a molecule of sulphuric acid separates water and forms an acid ethereal salt—that is there is produced an ethereal compound corresponding with acid salts. Thus, for example, the action of sulphuric acid, H2SO4, on ordinary alcohol, C2H5OH, gives water and sulphovinic acid, C2H5HSO4—that is, sulphuric acid in which one atom of hydrogen is replaced by the radicle C2H5 of ethyl alcohol, SO2(OH)(OC2H5), or, what is the same thing, the hydrogen in alcohol is replaced by the radicle (sulphoxyl) of sulphuric acid, C2H5O.SO2(OH).